Known to the art are micromachining techniques for providing integrated pressure sensors made of semiconductor material. Said sensors present numerous advantages in terms of low cost, high degree of functionality and reliability, good signal-to-noise ratio, integrability with memory circuits to obtain smart sensors, and high reproducibility. Semiconductor pressure microsensors present on the market are essentially based upon two physical effects: the piezoresistive effect, whereby the deflection of a silicon membrane caused by the pressure unbalances a Wheatstone bridge provided with resistances diffused in the membrane; and the capacitive effect, whereby the pressure induces displacement of a membrane that constitutes the mobile electrode of a capacitor (thus determining variation of capacitance thereof). In what follows only pressure sensors that use the first effect, i.e., piezoresistive sensors, will be considered.
A method of manufacture of a piezoresistive pressure sensor of a known type is described, for example, in European Patent No. 822398 or in European Patent No. 1577656, the disclosures of which are incorporated by reference. The membranes of said sensors, in order to guarantee proper operation, must have a well-controlled homogeneous thickness and moreover must not present intrinsic mechanical stresses (of a tensile and compressive nature). A method of fabrication of a membrane designed for use in piezoresistive pressure sensors is, for example, described in U.S. Pat. No. 7,871,894, the disclosure of which is incorporated by reference.
One of the main disadvantages of piezoresistive pressure sensors is the high thermal drift, that they undergo as the temperature varies. In the absence of compensation, a variation temperature of approximately 10° C. in a piezoresistive pressure sensor can cause a non-negligible drift of the output signal, in particular for applications that require high sensitivity (e.g., medical applications such as artificial breathers, spirometers, altimeters, barometers, etc.). For this reason, it is necessary to equip these sensors with a system for compensation of thermal drift. One of the known methods comprises inserting said sensors in a transduction circuit based upon the Wheatstone bridge. This modality envisages inserting on the opposite branch of the bridge piezoresistive elements that are substantially the same as those mounted on the sensor element, but arranged so as not to undergo deformations linked to the pressure applied to the membrane. As the temperature varies, all the piezoresistors undergo approximately the same thermal drifts. In this way, the elements used for compensation rebalance the Wheatstone bridge, reducing the dependence of the output pressure signal upon the temperature of the transducer.
However, on account of variations of layout of the sensor, variations of morphology of the piezoresistors linked to spreads of the manufacturing process, local concentrations of impurities, and in general other conditions of physical mismatch, also in the case of Wheatstone-bridge connection of the piezoresistors, variations of the pressure signal at output from the bridge are not completely independent of temperature variations of the sensor. A further step of compensation of the variations of the output signal caused by the temperature is consequently necessary. For this purpose, it is necessary to acquire a signal correlated to the temperature to which the piezoresistors are subjected in use. There has consequently been proposed temperature sensors located in the proximity of the pressure sensor, adapted to be used for thermal compensation of the pressure sensor. Said double-sensor systems show a temperature gradient between the temperature sensor and the membrane of the sensor, on account of the different physical location. The time that elapses for stabilization of the temperature gradient is known as “warm-up drift”.
A solution to this problem has, for example, be proposed by Kuo Huan Peng, C. M. Tiang, and Yih Min Chang. “The temperature compensation of the silicon piezoresistive pressure sensor using the half-bridge technique”, Proc. SPIE 5343, 292 (2004), the disclosure of which is incorporated by reference. In this document, the output drift of the bridge due to the variation in temperature is minimized by means of auto-gain circuits (AGCs) for adjusting automatically the voltage supplied to the bridge. This solution presents, however, certain disadvantages. In particular, the size of the pressure sensor is considerably increased for housing the temperature-sensing circuit. In addition, a high number of pads are used for biasing correctly both the piezoresistors and the temperature-sensing circuit.
There exists a need in the art to provide a transducer equipped with a temperature sensor, and a method for sensing a temperature of said transducer.